I am like @gdon_2003 and set my 0 at center. I find it is far more consistent for me. I am rarely working with perfectly square material and if I am working from center then it does not matter if there are some slight irregularities in the material. Almost everything I make has waste built in and after the main carving I use the CNC to cut the part out so working from center works really well. There was also a thread a few days ago that discussed this and working from center can make sure you get the part looking exactly how you want. See this thread for some more discussion on it.